Well, I don't think a deity did it. But it is well established at this point that your brain makes a decision long before your conscious mind rationalizes it. ("Long before" being relative...in this case on the scale of time of synapses firing.) This has been observed using fMRI.

But you are correct that he is inferring behavior of ancient tribes based on behavior of modern (but isolated) ones. The assumption would seem to be that what a modern isolated tribe needs for survival is much like that of an ancient clan. They live without any infrastructure or technology to speak of (yes, spears are technically "technology", but hopefully you know what I mean), getting what they need from the land as hunter-gatherers. Just like the ancient tribes did. The unknown factor might be environmental stresses in a given region, and those could affect behavior. But otherwise one would expect such groups to adopt similar social conventions because they work (or at least are survival neutral). If some behavior had deleterious effects it would be discouraged by the clan members and/or selected against by lack of survival of the members (or entire clan).